Two-Dimensional Electron Gas at LaAlO3/SrTiO3 Interfaces

36In this chapter, we review the status and prospects of the two-dimensional electron gas (2DEG) at LaAlO₃/SrTiO₃ interfaces. We start by a brief introduction, and then turn to the fabrication of such heterostructures, emphasizing the significant role of fabrication conditions in its structural and physical properties. The main body of the review chapter is devoted to the understanding of abundant physical properties and origins of the oxide 2DEG. Subsequently, we will discuss the applications of the oxide 2DEG in oxide electronic devices. Finally, prospects and existing challenges in this field will be reviewed.

Strongly correlated complex oxides exhibit luxuriously abundant and exotic physical properties (such as high-temperature superconductivity [1,2], metal-insulator transition [MIT] [3,4], colossal magnetoresistance (MR) [5,6], electronic phase separation [7–9], ferroelectricity [10], and multiferroicity [11]) owing to their interacting and competing lattice, charge, spin, and orbital degrees of freedom. They have broad applications in both large scale (such as the Maglev train system, electricity grids, and high magnetic field generators) and various microscopic electronic devices (such as complementary metal-oxide-semiconductor, optoelectronic, and memory devices), as well as chemical catalysis, lithium batteries, and solar energy conversion.

Advances in the heteroepitaxy of complex oxides provide fantastic feasibility of fabricating epitaxial oxide heterostructures including strongly correlated oxides with atomic precision. In the last decade, interface electronics in epitaxial complex oxide heterostructures has been a research focus in oxide electronics. In fact, it had been significantly stirred by novel functionalities created at interfaces that do not exist in the bulk in nature [12]. For example, ferroelectricity emerges in SrTiO₃ thin films grown on DyScO₃ substrates owning to interface strain [13]; the bulk polarization of BiFeO₃ can be modulated by interface coupling in BiFeO₃/La_0.7Sr_0.3MnO₃ heterostructures [14]; and a magnetic proximity effect shows up in YBa₂Cu₃O₇/ La_0.7Ca_0.3MnO₃ superlattices [15].

Particularly, the discovery of a high-mobility two-dimensional electron gas (2DEG) at the interface between two complex oxide insulators SrTiO₃ (STO) and LaAlO₃ (LAO) [16] fires the field of interface electronics in oxides. On one hand, high-mobility 2DEGs based on conventional semiconductors such as Si and GaAs have been utilized in high-mobility transistors that have built the entire semiconductor industry and enabled the discovery 37of the integer and fractional quantum Hall effects. On the other hand, the 2DEG at the LAO/STO interface exhibits a variety of functional properties such as superconductivity [17], the magnetic Kondo effect [18], and electronic phase separation [9] that are unachievable in conventional semiconductor 2DEGs. Hence, the oxide 2DEG is of significant interest in fundamental science and technology.

In this chapter, we will review the status and prospects of the 2DEG at LAO/STO interfaces. We start by a brief introduction, and then turn to the fabrication of such heterostructures, emphasizing the significant role of fabrication conditions in its structural and physical properties. The body of the review chapter is devoted to the understanding of abundant physical properties and origins of this oxide 2DEG. Subsequently, we will discuss the application of the oxide 2DEG in oxide electronic devices. Finally, prospects and existing challenges in this field will be reviewed.